Because atmospheres containing submicron-sized sulfate or nitrate particles, and sulfur dioxide or nitrogen dioxide are considered potentially hazardous to health, studies are proposed which will assess the impact of these pollutants in combinations on pulmonary defense systems. Exposure of rodents to ambient or near-ambient concentrations of sulfate particles plus sulfur dioxide, and nitrate particles plus nitrogen dioxide will be evaluated to determine if these pollutants act additively or synergistically to inhibit mucociliary transport (P32-labelled Staphylococcus aureus), phagocytic ingestion (histologic localization of bacteria), and killing of phagocytized bacteria (measurements of bacterial viability).